With the help of a friend of mine, k-ray ( http://k-ray.netbox.be/?&jq=04010 ), and SnackPack ( http://www.linux-hacker.net/cgi-bin/UltraBoard/UltraBoard.pl?Action=ShowPost&Board=verytech&Post=114&Idle=&Sort=&Order=&Page=&Session= ) I was able to get a Rand McNally GPS unit working with the WP.
The GPS receiver has 4 wires (Tx, Rx, GND, and +5V) and would otherwise connect to a PC with rs-232 serial ports. The WP serial ports are TTL. Rather than using a Maxim part to convert the TTL to rs232 on the WP, then from rs232 to ttl on the GPS, we bypassed the TTL-> rs232 conversion all together.
This is the typical flow from PC to a serial device:
ttl -> rs-232 -> rs-232 -> ttl
Next, i performed a similar mod to the GPS unit with the phone cord like SnackPack did. I wired +5V, GND, Rx and Tx from CN15 on the WP to the 4 pins on the rj11 connector on the back (i don't use the modem.) Then, i tacked on the 4 wires from a phone cable (with rj11 connector on one end) onto the GPS receiver in the appropriate locations. After installing the Rand Mcnally software, I hooked the gps receiver to the phone jack on the back of the WP. The Rand McNally software detected the GPS unit and after the 3-5 minutes it took to locate and lock onto a satellite, i was up and running.
rs232 enables you to run long distances of cable with no transmission errors. The only draw back to this method i choose is that i'm limited the distance of cable between the GPS unit and the WP. Since I will be using this in my car, that won't become an issue.
My next step is to build a DC-DC power supply, and then mount the WP in the car. I have a USB nic to transfer mp3s to the WP with hard drive, and i have a kenwood aux adapter to feed the audio into my stereo.